Many earth moving vehicles use pressurized hydraulic fluid as a mechanism for performing work. For example, with a motor grader, an earth-engaging blade downwardly depending from a main frame may be lifted, rotated and tilted using hydraulic cylinders; while with an excavator, a boom arm may be articulated with first and second hinged arms and a bucket at the terminus of the arms, each being associated with a hydraulic cylinder to effect movement.
Another example is a loader. A loader can be of a wheeled or track-type variety and include a lift arm hinged to the loader, with a rotatable bucket or other implement hinged to an end of the lift arm. A lift cylinder may be associated with the lift arm and a tilt cylinder may be associated with the bucket or other implement. When it is desired to lift the arm, hydraulic fluid is directed to the lift cylinder, and when it is desired to rotate the bucket or implement, hydraulic fluid may be directed to the tilt cylinder.
In order to control the tilt and lift cylinders, currently available loaders typically provide a single pump source to direct hydraulic fluid from a tank or reservoir to one of the cylinders in serial fashion. The pumps used can be a fixed displacement type pump, in which case a control valve may be used to regulate the fluid being communicated to the cylinder. Alternatively, the pumps used can be a variable displacement type to enable same.
While effective, such an arrangement is limiting in that the tilt and lift cylinders cannot be independently and simultaneously used for most cases. For example, when digging, the operator would ideally be able to lift and tilt at the same time to facilitate the process. However, currently available loaders prioritize between the lifting and tilting functions, with tilting typically having priority over lifting. As a result, the operator is significantly limited in his or her ability to perform the task at hand, in that the operator has to continuously switch back and forth between using the tilt function and lift function to load the bucket.